1. Field of the Invention
The present invention relates to a dewatering process and more particularly to a belt for use in the dewatering process.
2. Related Art
In any industrialized country there currently exists the problem of the disposal of the vast and ever increasing amounts of sludge produced during the purification of drinking water and sewage and the dredging of waterways such as harbors, canals and drainage channels. Some sludge may be used as a fertilizer or landfill material, or may be incinerated, but the largest proportion of waste sludge must be dumped due to its contaminated nature, particularly where it has been recovered from areas of significant industrial activity. As the price for disposal of sludge rises so does the need to reduce the sludge weight and volume, in order to lower both dumping and transport costs.
Waste sludge typically has a very low solids content (less than 10 wt. %--the rest being water), so it makes sense to reduce the weight and volume by lowering the water content. Traditional gravitational techniques such as sedimentation or flotation are slow and can only achieve maximum solids content in the order of 12 wt. %. More recent methods include the use of chamber filter presses, wire belt presses and centrifugal techniques. The former requires a large amount of flocculating agent to be added, whilst the latter two are restricted by the relatively low dwell time of the sludge in the dewatering areas. All are particularly unsuitable for fine, highly colloidal sludges, where a significant proportion of the solid particles can not be retained without the use of a much finer filter medium, thereby lowering the permeability of the sludge bed and leading to maximum achievable solids content of about 15 wt. % for such sludge types.
The use of electro-osmosis as a dewatering technique is described in DE 124509. A material is dispersed in a polar solution having a charged surface. Therefore the electrolyte in the immediate vicinity of the charged surface possesses a net excess opposite charge due to the electro-static attraction forces of the ions of opposed polarity. In the region of the charged surface a so-called diffuse double layer is formed, whereby some of the ions are firmly bonded at said surface and are removed from the equilibrium arising from the tendency of particles in the highly. concentrated surface region of the liquid to diffuse into the less concentrated bulk interior. This results in a concentration gradient of ions which drops off rapidly away from the charged surface. In order for these firmly bonded ions to become mobile once again, a potential needs to be applied--the so-called zeta potential. By keeping the sludge matrix between the electrodes, the rate of dewatering at the cathode will be accelerated upon the application of an electric field corresponding to at least the value of the zeta potential, since the ions are then able to move freely to and from the disperse phase.
The effectiveness of mechanical dewatering techniques is dependent upon the permeability of the medium to be dewatered, since the applied force (pressure for wire belt or chamber filter presses, gravitational acceleration for decanter centrifuges) compacts the bed of material particles. Hence the porosity of the bed is reduced and therefore the flow of water from the bed is reduced. For electro-osmosis the liquid flow is independent of the bed porosity. Here it is the electrical conductivity of said disperse phase that is the key factor and therefore low bed porosities will not reduce the effectiveness of water removal. Dewatering times can be reduced with respect to mechanical techniques by applying a potential difference across the sludge bed. This reduction in time results in a net energy saving.